This invention relates generally to radio frequency filters and more particularly to duplexer radio frequency filters utilizing harmonic rejection to improve ultimate rejection outside of the bandpass regions.
In radio communication equipment employing both a receiver and a transmitter which may be operated simultaneously on separate but closely spaced frequencies and on a single antenna, a special radio frequency (RF) filter is generally employed to isolate the transmitter signal from the signal to be received by the receiver. The difference in power between the two signals typically is many orders of magnitude thus exceeding the dynamic range capability of linear receiver amplifiers which are not protected by a filter. Furthermore, consideration must also be given to the effects of noise and harmonics of each signal and the nonlinear effects of elements within the path of the two signals when designing a duplexer filter. These considerations have been addressed previously in earlier implementations of duplexers such as those described in U.S. Pat. Nos. 3,293,644 and 3,728,731.
Recent developments in ceramic resonators have produced duplexer filters which have significant advantages in size, cost, and performance over earlier implementations. Such filters are described further in U.S. Pat. application No. 656,121 ("Single Block Dual-Passband Ceramic Filter", filed on behalf of Kommrusch on Sept. 27, 1984), U.S. Pat. No. 890,682 (Multiple Resonator Component-Mountable Filter", filed on behalf of Moutrie et al. on July 25, 1986), and U.S. Pat. No. 890,686 ("Multiple Resonator Dielectric Filter", filed on behalf of Green et al. on July 25, 1986). Transmission line structures, which are the primary technology of these dielectrically loaded filters, have periodic frequency responses which produce passbands at frequencies related to the odd harmonics of the desired passband frequency (flyback). This flyback can result in spurious signal detection in the receiver or in the transmission of signals at undesired frequencies from the transmitter. Previous attempts at controlling flyback response in duplexing schemes have utilized a separate harmonic filter component between the duplexer filter and the common antenna.